While the majority of human CD4+ cells express CD28+, CD8+ T cells have been shown to be heterogeneous in their expression of CD28. This heterogeneity has been used to distinguish cytotoxic and suppressor T cells, with CD8+/CD28+ cells classified as CTLs and CD8+/CD28- cells described as ?suppressor cells.? This distinction is made on the basis of the ability of CD8+/CD28+ T cells to mediated CTL and of CD8+/CD28- T cells to suppress lymphocyte proliferative responses to alloantigens, CTL activity or B cell. In addition, many CD8+/CD28- T cells also appear to express CD11, CD57 and, at least, one of the NK cell-associated killer inhibitory receptors (KIR) for self MHC-alleles. Significant proportions of these cells also express NKR-P1. Recent experimental evidence suggests that, in contrast to CD8+CD28+ T cells, the presence of CD8+/CD28- T cells is actually inhibitory toward the host response to viruses or tumors mediated by other cells (i.e., B cells, NK cells and CTLs). The data on aging and the percentage of CD28 expressing T cells are limited. A number of human studies have demonstrated that the percentage and number of CD3+/CD8+/CD28- cells increases with age. It appears that the age-related expansion of human CD8+/CD28- T cells is driven solely by an increase in CD57+ cells. Although CD8/CD28- T cells have enhanced suppressor cell activity in relatively young humans, it is not known whether they do so in the aged. However, it has been hypothesized that the expansion of these cells is a marker of immunosenescence and, in elderly humans, the appearance of these cells negatively predicts the effectiveness of influenza virus vaccination. The focus of this study is to establish primary cultures of CD28-/CD28+ and CD57+/CD57- T cells from rodents and humans of different ages and to examine the immunoregulatory role and the cytokine profiles of these cells in vitro immune responses of young, middle-aged and old animals, and of these populations. As the number of these cells are dramatically increased during autoimmune diseases including arthritis, AIDS, various hematological malignancies and aging, we believe that further defining the precise role(s) of these populations in aged hosts and autoimmune models will yield valuable results which may lead to new immunotherapies and adjuvants for aged and diseased individuals.